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The principals of transportation of natural gas work as follows:
To avoid the expense of laying pipelines, an alternative method was introduced that was the transportation of natural gas in liquid form. In this method natural gas is cooled up to -260oF to change its state from gase to liquid this cooling reduces the volume of natural gas about 600 times as that of in gaseous state.
Once the gas is in liquid form it can be transported easily as a liquid hydrocarbon from the remote producing wells to the area from where pipelines can easily be accessed (LNG terminals).
LNG terminals are sites for unloading LNG from transporting vehicles. These LNG terminals are equipped with the facilities for unloading, regassification, tanking and metering of natural gas. At these terminals natural gas is converted back into the gaseous state by heating it up to 0oc and distributed along the pipeline network. LNG terminals can be located at onshore or offshore areas. But mostly offshore locations are preferred because of marine transportation and less impact of Regasification on environment. One more advantage is that offshore Regasification units uses the sea water as a heating media for liquefied natural gas. Offshore Regasification units either be fixed or floating.
In case of floating Regasification units specialized vessels are used for the storage and Regasification of LNG. There are two types of vessels that can be used as a floating Regasification units i.e. FSRU (floating storage and Regasification unit) and SRV (shuttle Regasification unit). Both units are performs same functions but with different operational procedures and design.
FSRU is just like a conventional LNG vessel that has ability to carry on Regasification and storage processes at any location. It mostly operates at a terminal with specific design that is constructed over a protected locations and carry on the operations of receiving, regassifying and injecting or distributing natural gas.
In complete cycle of LNG production and uses there are three main components that are natural gas producing wells, liquefaction facility, LNG Regasification terminal and consuming facility. Natural gas producing wells are oil and gas wells that are drilled either onshore or offshore to produce the natural gas from petroleum traps. Once the natural gas is produced it is sent to liquefaction facility where its temperature is decreased to change its state from gaseous state to liquid state these facilities are mostly located onshore either near giant gas producing fields or gas is supplied via pipelines. But another approach to make it cost effective floating a new advancement that can be done is floating liquefaction facilities just like Regasification facilities. Once the natural gas is liquified it is transported in same manner like oil or other liquid hydrocarbons in tankers and can be transported globally.
The next step after the transportation of liquid gas is regasification of LNG that is the again conversion of liquid gas that is done at Regasification terminal these are mostly located offshore in floating vessels because of their cost effectiveness and environmental impacts only very less regassifying terminals are located in onshore areas. LNG is received at regassifying terminals via transporting medium that are LNG tankers. Once the LNG is received it is unloaded from the transporting vehicle by using specific hoses known as unloading hoses that avoids any escape of natural gas. During unloading gas is metered and later on stored in the tanks present at that facility. While transportation and unloading of LNG small amount of liquified natural gas is reconverted into gaseous form due to natural evaporation that is unavoidable. This evaporated gas is known as boil off gas.
Regasification facilities are provided with low pressure pumps, high pressure pumps and recondenser. Low pressure pumps are installed within storage tanks that conveys LNG to recondenser where this cold LNG is brought in contact with compressed BOG obtained from storage tanks causing the condensation of boil off gas back into liquid state before passing LNG to the high pressure pumps. Small amount of boil off gas is consumed as a fuel FSR unit. Recondensers are the new addition in FSRU that reduces the gas wastage making it more economical. Before this, floating vessels were not equipped with recondenser and BOG left after the fuel consumption was burnt in boilers and produced steam was disposed into the ocean in form of condensate.
The function of high-pressure pumps is to raise the LNG pressure from tank pressure according to the customer demand. Storage tank pressure of LNG is normally 5 bar g and customer requirement is variable according to the nature of use. For example client requirement for power plant is along 50 bar g pressure and requirement for gas network is about 100 bar g. So, the vaporization of gas is done at desired pressure. Nearly all natural gas consuming infrastructure is installed in onshore areas. So, delivery of gas is done via pipeline networks and export of gas is metered for financial records.
To make this production and consumption cycle more economical it is proposed to make storage and power generation vessels so that distribution cost might be reduced. The power generation and storage facilities might be placed on FSRU or isolated barges. This practice might be very cost effective in linking the Regasification units to the land-based gas consuming sites. Because these sights are far away from LNG terminals and pipelines network demands huge cost. Because of growing demand of natural gas cost effective strategy should be adopted to meet energy requirements. For this purpose, shifting of power generation land infrastructure to floating vessels is quite good option. Because it would be very useful if in future the exploration industry moves towards the exploitation of natural gas from gas hydrates present in the ocean.